1. Field of the Invention
The present invention generally relates to an ocular blood-flow meter for measuring the flow rate of blood in a blood vessel of a patient""s eye.
2. Description of the Related Art
An ocular blood-flow meter utilizing the Doppler effect determines the flow rate of blood in the following manner. A laser beam is applied to a blood vessel of a subject eye, and the light scattered and reflected by the blood vessel is received by a photodetector. Then, an interference signal of a Doppler shift component, i.e., the light scattered and reflected by the blood flow, and the light scattered and reflected by a stationary blood-vessel wall is detected. Upon analyzing the frequency of the interference signal, the blood-flow rate is determined. That is, the blood-flow rate (maximum rate Vmax) is determined according to the following equation:
Vmax={xcex/(nxc2x7xcex1)}xc2x7∥xcex94fmax1|xe2x88x92|xcex94fmax2∥/cosxcex2xe2x80x83xe2x80x83(1)
wherein xcex94fmax1 and xcex94fmax2 indicate the maximum frequency shifts calculated from the received-light signals received by two photodetectors; xcex represents the wavelength of the laser light; n designates the index of refraction of a portion to be examined; a indicates the angle between the two light-detecting optical axes within the eye; and xcex2 represents the angle between the plane formed by the two light-detecting optical axes and the velocity vector of the blood flow. By measuring the flow rates from the two directions as discussed above, contributions due to the directions of incidence of the measuring beams are canceled, thereby making it possible to measure the flow rate of blood at a certain portion on the eye fundus. By matching the line of intersection between the plane formed by the two light-detecting optical axes and the eye fundus to the angle xcex2, xcex2 becomes 0 degrees, thereby measuring the true maximum flow rate.
In measuring the flow rate with an ocular blood-flow meter, if the relative position of an optical system of the ocular blood-flow meter with respect to a portion of the eye to be examined is changed due to involuntary eye movement, it becomes difficult to perform precise measurements. In order to solve this problem, U.S. Pat. No. 4,856,891 discloses a tracking device. As described in this patent, a beam of light is applied from a tracking light source to a subject vessel, and the resulting blood-vessel image is captured by a charge-coupled device (CCD) camera. Then, the tracking device performs tracking by scanning the beam of light from the tracking light source so that the blood vessel image can be stabilized at a fixed position of the CCD camera in accordance with the eye movement.
However, the maximum value xcex94fmax1 of the Doppler shift in equation (1) is detected as an interference signal between the Doppler component shifted by the flow of blood and the stationary vessel wall. Thus, the maximum frequency shift xcex94fmax obtained by analyzing the frequencies lacks sign information since what is measured is |xcex94fmax|. In measuring the flow rates in different portions of the eye fundus, the signs of the maximum frequency shifts xcex94fmax1 and xcex94fmax2 may both be positive, or they may both be negative, or one value may be positive and the other value may be negative. Accordingly, the maximum flow rate Vmax cannot be determined for some portions according to equation (1).
U.S. Pat. No. 5,640,963 discloses an eye-fundus-blood-flow meter provided with a mechanism for switching the directions of incidence of the light beams in order to precisely measure the flow rate of blood regardless of the eye-fundus-vessel portion measured or the direction of the eye fundus vessel. However, there is still room for improvement in this flow meter. That is, when measurements are performed in a single direction of incidence, the patient""s eyelashes may eclipse the beam of light, or a displacement in the alignment or blinking, or a poor fixation point may be selected, thereby causing a failure to perform correct measurement in this direction of incidence. Due to the incorrect measurement in this direction, even if a correct measurement is performed in the other direction, it is determined that measurements are incorrectly performed in both directions, thereby wasting the measurement of correctly measured paths. Additionally, after performing re-alignment, measurements must be performed once again in both directions of incidence, thereby subjecting a patient to a long measurement time.
Accordingly, it is an object of the present invention to improve a conventional eye-fundus-blood-flow meter, and more specifically, to provide a highly precise and easy-to-use ocular blood-flow meter in which it can be easily determine whether measurements should be repeated.
In order to achieve the above objects, according to one aspect of the present invention, there is provided an ocular blood-flow meter comprising an optical system configured and positioned to apply measuring light to a blood vessel of a subject eye, and to receive light scattered by the blood vessel of the subject eye. The meter also comprises a mechanism configured and positioned to change the direction in which the measuring light is applied to the blood vessel or the direction in which the scattered light is received by at least a portion of the optical system so as to enable a plurality of measurements of the blood flow in the blood vessel using measuring light applied to the blood vessel in different directions or using scattered light received by at least a portion of the optical system in different directions. The meter further comprises a device configured and positioned to receive the scattered light from the optical system. The device outputs a received-light signal containing information on blood flow in the blood vessel in response to receiving the scattered light from the optical system. In addition, the meter comprises a controller connected to the device to receive the received-light signal and configured to perform the plurality of measurements of the blood flow in the blood vessel using the received-light signal generated from measuring light being applied to the blood vessel in different directions and scattered by the blood vessel or generated from the scattered light received by at least a portion of the optical system in different directions. The controller is also configured to perform a re-measurement operation to re-measure the blood flow in the blood vessel using a received-light signal generated from measuring light being applied to the blood vessel in a desired direction and then scattered by the blood vessel or generated from the scattered light received by at least a portion of the optical system in a desired direction in response to an instruction by an operator to perform a re-measurement operation in the desired direction. In addition, the meter comprises an input device. The input device is electrically coupled to the controller. The input device is configured to enable an operator to select a re-measurement operation to re-measure the blood flow in the blood vessel using a received-light signal generated from measuring light being applied to the blood vessel in a desired direction and then scattered by the blood vessel or generated from the scattered light received by at least a portion of the optical system in a desired direction. The input device also is configured to instruct the controller to perform the selected re-measurement operation selected by the operator.
According to another aspect, the present invention that achieves at least one of these objectives relates to an ocular blood-flow meter comprising an optical system configured and positioned to apply measuring light to a blood vessel of a subject eye, and to receive light scattered by the blood vessel of the subject eye. The meter also comprises a mechanism configured and positioned to change the direction in which the measuring light is applied to the blood vessel or the direction in which the scattered light is received by at least a portion of the optical system so as to enable a plurality of measurements of the blood flow in the blood vessel using measuring light applied to the blood vessel in different directions or using scattered light received by at least a portion of the optical system in different directions. In addition, the meter comprises a controller configured to perform the plurality of measurements of the blood flow in the blood vessel using the measuring light being applied to the blood vessel in different directions and scattered by the blood vessel or using the scattered light received by at least a portion of the optical system in different directions. The controller is also configured to perform a re-measurement operation to re-measure the blood flow in the blood vessel using measuring light being applied to the blood vessel in a desired direction and then scattered by the blood vessel or using scattered light received by at least a portion of the optical system in a desired direction. The controller is also configured to determine whether a re-measurement operation is required. The meter further includes an output device connected to the controller. The output device is configured to present information to an operator indicating whether re-measurement is required in response to the controller determining that a re-measurement operation is required.
According to still another aspect, the present invention that achieves at least one of these objectives relates to an ocular blood-flow meter comprising optical means for applying measuring light to a blood vessel of a subject eye, and for receiving light scattered by the blood vessel of the subject eye. The meter also includes direction-changing means for changing the direction in which the measuring light is applied to the blood vessel or the direction in which the scattered light is received by at least a portion of the optical means so as to enable a plurality of measurements of the blood flow in the blood vessel using measuring light applied to the blood vessel in different directions or using scattered light received by at least a portion of the optical means in different directions. In addition, the meter includes signal-outputting means for outputting a received-light signal containing information on blood flow in the blood vessel in response to receiving the scattered light from the optical means. Also, the meter includes control means for performing the plurality of measurements of the blood flow in the blood vessel using the received-light signal generated from measuring light being applied to the blood vessel in different directions and scattered by the blood vessel or generated from the scattered light received by at least a portion of the optical means in different directions. The control means comprises means for performing a re-measurement operation to re-measure the blood flow in the blood vessel using a received-light signal generated from measuring light being applied to the blood vessel in a desired direction and then scattered by the blood vessel or generated from the scattered light received by at least a portion of the optical means in a desired direction in response to an instruction by an operator to perform a re-measurement operation in the desired direction. The meter also includes input means for enabling an operator to select a re-measurement operation to re-measure the blood flow in the blood vessel using a received-light signal generated from measuring light being applied to the blood vessel in a desired direction and then scattered by the blood vessel or generated from the scattered light received by at least a portion of the optical means in a desired direction. The input means comprises means for instructing the control means to perform the selected re-measurement operation selected by the operator.
According to still another aspect, the present invention that achieves at least one of these objectives relates to an ocular blood-flow meter comprising optical means for applying measuring light to a blood vessel of a subject eye, and for receiving light scattered by the blood vessel of the subject eye. The meter also comprises direction-changing means for changing the direction in which the measuring light is applied to the blood vessel or the direction in which the scattered light is received by at least a portion of the optical means so as to enable a plurality of measurements of the blood flow in the blood vessel using measuring light applied to the blood vessel in different directions or using scattered light received by at least a portion of the optical means in different directions. The meter further includes control means for performing the plurality of measurements of the blood flow in the blood vessel using the measuring light being applied to the blood vessel in different directions and scattered by the blood vessel or using the scattered light received by at least a portion of the optical means in different directions. The control means comprises means for performing a re-measurement operation to re-measure the blood flow in the blood vessel using measuring light being applied to the blood vessel in a desired direction and then scattered by the blood vessel or using scattered light received by at least a portion of the optical means in a desired direction. The control means also comprises means for determining whether a re-measurement operation is required. The meter further comprises output means for presenting information to an operator indicating whether re-measurement is required in response to the control means determining that a re-measurement operation is required.
According to still another aspect, the present invention that achieves at least one of these objectives relates to a method of measuring ocular blood flow in an ocular blood vessel comprising the steps of applying measuring light to a blood vessel of a subject eye, receiving light scattered by the blood vessel of the subject eye, changing the direction in which the measuring light is applied to the blood vessel in the applying step or the direction in which the scattered light is received in the receiving step so as to enable the performing of a plurality of measurements of the blood flow in the blood vessel using measuring light applied to the blood vessel in different directions or using scattered light received in different directions, outputting a received-light signal containing information on blood flow in the blood vessel generated from the scattered light received in the receiving step, performing the plurality of measurements of the blood flow in the blood vessel using the received-light signal generated from measuring light being applied to the blood vessel in different directions and scattered by the blood vessel or generated from the scattered light received in different directions, and performing a re-measurement operation to re-measure the blood flow in the blood vessel using a received-light signal generated from measuring light being applied to the blood vessel in a desired direction and then scattered by the blood vessel or generated from the scattered light received in a desired direction in response to an instruction by an operator to perform a re-measurement operation in the desired direction.
According to still another aspect, the present invention that achieves at least one of these objectives relates to a method of determining the ocular blood flow in an ocular blood vessel comprising the steps of applying measuring light to a blood vessel of a subject eye, receiving light scattered by the blood vessel of the subject eye, changing the direction in which the measuring light is applied to the blood vessel in the applying step or the direction in which the scattered light is received in the receiving step so as to enable a plurality of measurements of the blood flow in the blood vessel using measuring light applied to the blood vessel in different directions or using scattered light received in different directions, performing the plurality of measurements of the blood flow in the blood vessel using the measuring light being applied to the blood vessel in different directions and scattered by the blood vessel or using the scattered light received in different directions, performing a re-measurement operation to re-measure the blood flow in the blood vessel using measuring light being applied to the blood vessel in a desired direction and then scattered by the blood vessel or using scattered light received in a desired direction, determining whether a re-measurement operation is required, and presenting information to an operator indicating whether re-measurement is required in response to the determining step determining that a re-measurement operation is required.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.